The present invention relates to a method and to an apparatus for rewinding a thread.
When rewinding a thread, e.g. from a spinning cop onto a winding bobbin, the thread is placed by a thread laying unit on the bobbin surface. The winding bobbin is driven by a drive motor.
In practice, where several cops are successively rewound, a maximum production rate is sought. An increase in the production rate is obtained by increasing the winding speed without any increase in the number of thread breaks. If the winding speed exceeds a certain limit, there is an increase in the number of thread breaks and as a result an upper production rate limit is set. The higher the winding speed, the greater the tension in the thread, which explains the increase in thread breaks on increasing the winding speed.
If the winding speed is kept constant, it can be observed that the thread tension at the start of the rewinding of a cop is low. This is followed by a slow increase in the thread tension and towards the end of the rewinding process it rises rapidly. To prevent thread breaks occurring in the final phase of the rewinding process, the winding rate throughout the rewinding process is reduced to such an extent that during the final phase of the rewinding process, which is much shorter than the preceding rewinding period, a minimum number of thread breaks occurs. However, the consequence of this is that throughout said greater rewinding period, rewinding takes place slowly. Thus, in order to increase the production rate, the speed at which the thread is removed from the spinning cop is high. However, due to the constructional characteristics of spinning cops, the unwinding speed of the thread at the start of cop (full cop) and therefore the thread tension cannot be made too high, because otherwise complete layers or loops of thread would be drawn off.
Thus, to increase the production rate, it must be possible to modify the winding speed during the drawing off of the thread from a cop or spinning cop so as to ensure that throughout the rewinding process the thread tension is as near as possible, but remains below, a limit above which thread breaks frequently occur.
To achieve this, it is known to constantly adjust the speed of the drive motor in response to certain factors, which are e.g. the thread speed and thread tension. The magnitude of the thread tension can be controlled as a function of the winding-up time by a corresponding program, the diameter of the winding bobbin, the cop unwinding time or a combination of these quantities.
In order to achieve this type of control, it is e.g. known to make use of a thread tension meter, which continuously measures the thread tension. In the case of deviations between a constant or programmed adjustable desired or rated value and the actual thread tension value, the drive motor speed is adjusted in such way that the deviation is reduced. In the case of short-term deviations, it is possible to control an electronically operable thread braking means with the aid of an electronic control system in such a way that the deviation is reduced. The control signal used for this purpose can additionally be compared with a given desired value. When deviations occur between the instantaneous control signal and the desired value, the drive motor speed is adjusted with the aid of the electronic control system so that said deviation is reduced. However, this known method still does not permit the presently required increase in the production rate in the case of partly unwound cops.